1. Field of the Invention
The present invention relates generally to uses of ultrasonics in medical technology applications, more particularly to a method and apparatus for performing presurgical and surgical procedures using high-intensity focused ultrasound.
2. Description of Related Art
Studies in the use of ultrasound--sound with frequency above 20,000 Hz, the upper limit of human hearing--began in the early 1940's [see e.g., A New Method for the Generation and Use of Focused Ultrasound in Experimental Biology, Lynn et al., Focused Ultrasound in Experimental Biology, Journal of General Physiology, 1943, pp. 179-193]. It is widely accepted that the first refined system for the use of ultrasound in the medical arts was developed by William and Francis Fry at the University of Illinois, Urbana, in the 1950's (a paper was published as part of the Scientific Program of the Third Annual Conference of the American Institute of Ultrasonics in Medicine, Washington D.C., Sep. 4, 1954, pp. 413-423).
In the main, research and development has been concerned with diagnostic and therapeutic applications. Therapeutic ultrasound refers to the use of high intensity ultrasonic waves to induce changes in tissue state through both thermal effects--induced hyperthermia--and mechanical effects--induced cavitation. High frequency ultrasound has been employed in both hyper-thermic and cavitational medical applications, whereas low frequency ultrasound has been used principally for its cavitation effect. Diagnostic medical ultrasonic imaging is well known, for example, in the common use of sonograms for fetal examination.
Various aspects of diagnostic and therapeutic ultrasound methodologies and apparatus are discussed in depth in an article by G. ter Haar, Ultrasound Focal Beam Surgery, Ultrasound in Med. & Biol., Vol. 21, No. 9, pp. 1089-1100, 1995, and the IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, November 1996, Vol. 43, No. 6 (ISSN 0885-3010). The IEEE journal is quick to point out that: "The basic principles of thermal effects are well understood, but work is still needed to establish thresholds for damage, dose effects, and transducer characteristics . . . " Id., Introduction, at page 990.
In high-intensity focused ultrasound (HIFU) hyperthermia treatments, intensity of ultrasonic waves generated by a highly focused transducer increases from the source to the region of focus where it can reach a very high temperature, e.g. 98.degree. Centigrade. The absorption of the ultrasonic energy at the focus induces a sudden temperature rise of tissue--as high as one to two hundred degrees Kelvin/second--which causes the irreversible ablation of the target volume of cells, the focal region. Thus, for example, HIFU hyperthermia treatments can cause necrotization of an internal lesion without damage to the intermediate tissues. The focal region dimensions are referred to as the depth of field, and the distance from the transducer to the center point of the focal region is referred to as the depth of focus. In the main, ultrasound is a promising non-invasive surgical technique because the ultrasonic waves provide a non-effective penetration of intervening tissues, yet with sufficiently low attenuation to deliver energy to a small focal target volume. Currently there is no other known modality that offers noninvasive, deep, localized focusing of non-ionizing radiation for therapeutic purposes. Thus, ultrasonic treatment has a great advantage over microwave and radioactive therapeutic treatment techniques.
A major issue facing the use of HIFU techniques is cavitation effects. In some quarters, it is recognized that cavitation can be used advantageously. See e.g., Enhancement of Sonodynamic Tissue Damage Production by Second-Harmonic Superimposition: Theoretical Analysis of Its Mechanism, Unmemura et al. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 43, No. 6, 11/96 at page 1054. Cavitation can occur in at least three ways important for consideration in the use of ultrasound for medical procedures. The first is gaseous cavitation, where dissolved gas is pulled from solution during a negative pressure phase of an acoustic wave. The second is vaporous cavitation due to the negative pressure in the negative pressure phase becoming low enough for a fluid to convert to its vapor form at the ambient temperature of the tissue fluid. The third is where the ultrasonic energy is absorbed to an extent to raise the temperature above boiling at ambient pressure. At lower frequencies, the time that the wave is naturally in the negative pressure phase is longer than at higher frequencies, providing time for gas or vapor to come out of the fluid. All other factors being equal, a lower frequency will have a lower intensity level for cavitation than a higher frequency. Higher frequencies are more rapidly absorbed and therefore raise the temperature more rapidly for the same applied intensity than a lower frequency. Thus, gaseous and vaporous cavitation are promoted by low frequencies and boiling cavitation by high frequency.
For HIFU applications it has been found that ultrasonically induced cavitation occurs when an intensity threshold is exceeded such that tensile stresses produced by acoustic rarefaction generates vapor cavities within the tissue itself. Subsequent acoustic compressions drive the cavities into a violent, implosive collapse; because non-condensing gases are created, there are strong radiating pressure forces that exert high shear stresses. Consequently, the tissue can shred or be pureed into an essentially liquid state. Control of such effects has yet to be realized for practical purposes; hence, it is generally desirable to avoid tissue damaging cavitation whenever it is not a part of the intended treatment.
Another problem facing the designer of ultrasound medical devices is that the attenuation and absorption rate of ultrasound in tissue is known to exponentially increase in proportion to the frequency. In other words, a very high frequency, e.g., 30 MHz wave would be absorbed nearly immediately by the first tissue it is applied to. Yet, lower frequencies, e.g., 30 KHz-60 KHz, are associated with cavitation effects because of the longer rarefaction time periods, allowing gaseous vapor formation. Thus, the effect of ultrasound energy is quite different at a frequency of 30 KHz versus 30 MHz. The rate of heat generation in tissue is proportional to the absorption constant. For example, for the liver, the attenuation constant is approximately 0.0015 at 30 KHz, but is approximately 0.29 at 3 MHz. Therefore, all other variables being equal, the heat generated in liver tissue is about 190 times greater at 3 MHz than at 30 KHz. While this means hyperthermia can be achieved more quickly and to a much greater level with high frequencies, the danger to intervening tissue between the transducer and the focal region is much more prevalent.
Thus, there is a continuing need in the field for means of improving heating penetration, spatial localization, and dynamic control of ultrasound for medical applications, along with the discovery of new methodologies of their use.
An even less explored field of ultrasound use is as a direct surgical tool for non-invasive surgical procedures. While ultrasound has been used as a electromechanical driver for cutting tool implementations (see e.g., U.S. Pat. No. 5,324,299 to Davison et al. for an ultrasonic scalpel blade, sometimes referred to as a "harmonic scalpel"), the use of ultrasonic radiation directly in a device for performing presurgical and surgical procedures, rather than therapeutic procedures, has been limited. An ultrasonic diagnostic and therapeutic transducer assembly and method of use for ophthalmic therapy is shown by Driller et al. in U.S. Pat. No. 4,484,569. The acoustic coupler in the Driller device is a fluid-filled, conical shell, mounted to a transducer apparatus and having a flexible membrane across the apertured, distal end of the cone. Depth of focus is controlled by changing the aperture size and the spacing from the focal point of a fixed focal point transducer by using cones of varying axial length. The problem with this construct is that the focal point is fixed and that the cones must be manually changed (compare Driller's FIG. 2 and FIG. 5).
During invasive surgery, an obvious primary problem is bleeding. The most common surgical technique in the state of the art for coagulating bleeding vessels is to apply an electrical cauterizing probe to the bleeding site (see, e.g., U.S. Pat. No. 4,886,060, to Wiksell for an ultrasonically driven, oscillating knife having a means for emitting high-frequency electrical energy which induces heat in the tissue). However, if a bleeding vessel is more than about 1.5 mm in diameter, or in an organ such as the liver, which is highly vascularized and where uncontrolled hemorrhage is the primary cause of death in hepatic trauma, direct electro-cauterization is ineffective. A more complicated technique of clamping of a large blood vessel and electrical cauterization via the clamp or with laser light can sometimes be effective. A major problem that is not solved with either electrical or laser cauterization techniques is the control of a rapidly bleeding vessel because the blood egress is often sufficiently large enough to carry the heat away before coagulation or tissue necrosis is accomplished. In liver surgery, neither is effective. Moreover, organs such as the liver and spleen are subject to bleeding profusely from cracks in the parenchyma, which is usually diffuse and non-pulsatile due to the large number of small vessels. In another example, the control of bleeding is the most important variable in determining the length of neurosurgical craniotomy procedures.
Another important application in need of technologically advanced medical treatment is for emergency hemorrhaging situations, e.g., an accidently severed femoral artery, massive internal bleeding, or puncture wounds due to bullets, knives, or automobile accidents. Prompt stemming of visible hemorrhaging is literally a matter of life or death. Standard procedure to arrest visible bleeding is to maintain pressure on the puncture site until coagulation is sufficient to stem the flow of blood. Without sophisticated hospital equipment and invasive surgery, the problem of internal bleeding lacks suitable emergency treatment devices.
The present invention meets the various needs in the field of technology by presenting a method and apparatus using high intensity focused ultrasound for inducing coagulative necrosis and hemostasis that can be used in presurgical procedures such that substantially bloodless surgery can be achieved. The present invention further provides methods and apparatus using high intensity focused ultrasound for stemming hemorrhaging in emergency situations or with organs where tradition methodologies are ineffective.